Users of the mobile devices (laptops, palmtops, mobile phones, smart phones, multimedia phones, portable media players, GPS units, mobile gaming systems, etc.) may have applications installed that periodically receive notification messages from notification services. For example, such applications include “push” e-mail services (e.g., MobileMe, Microsoft Exchange, ActiveSync, Push-IMAP, Yahoo! Push, etc.), or other push services (e.g., update/upgrade services, news services, web blog services, podcast services, social networking services, or other types of services where notification messages may be sent). Notification messages typically represent events of interest, which are typically defined by the applications (e.g., new e-mail indicator, new news item indicator, new podcast indicator, change of on-line status of a social networking friend, etc.).
The increase in the use of mobile devices magnifies the complexity of routing notification messages to those devices. One problem is that mobile devices are not inherently addressable; for example, there is currently no mobile version of IPv6. In other words, mobile devices do not have their own IP addresses in the same way that a desktop computer, or even a laptop computer, has an IP address. Furthermore, mobile devices are sometimes logically behind a service provider firewall that likely employs network address translation (NAT). Such firewalls are applicable both within the cellular context and the wi-fi context. Given that mobile devices are not inherently addressable, it is difficult to route messages to a mobile device, particularly on a large scale.
Scalability becomes a particular issue within the context of notification messages sent to mobile devices as the number of mobile devices connected to a network increases. For example, a network device that connects to mobile devices can typically manage device connections for thousands of mobile devices at a time. Thus, to accommodate hundreds of millions of mobile devices would require hundreds of thousands of network server devices to manage the connections and the routing of messages to those hundreds of millions of devices. Of course, hundreds of thousands of network server devices would be both cost prohibitive and very complex to implement. Additionally, static routing of messages using network server devices are often not failsafe or fault tolerant, meaning that if a network device goes down, it is possible that notification messages will not be able to reach certain mobile devices.